There are conventional methods for removing nitrogen oxide contained in gas such as an ammonia denitration method and an urea denitration method for removing nitrogen oxide contained in a combustion gas for a boiler or a power generation facility such as a gas turbine or a gas engine etc. Further, there is a three-way catalyst method for removing nitrogen oxide contained in an exhaust gas of an automobile by using hydrocarbon contained in the exhaust gas as a reducer. In the removal of nitrogen oxide contained in the exhaust gas by the above method, the hydrogen oxide is reduced to nitrogen gas by using an energy of the exhaust gas at several hundred Celsius degrees. Further, the concentration of the nitrogen oxide in the combustion gas or the exhaust gas is generally several hundred ppms.
When the above nitrogen oxide removal method used for combustion gas or exhaust gas is applied as a method for removing nitrogen oxide contained in air, there is a problem of a low reaction rate due to a low concentration of nitrogen oxide contained in the air. In this case, there is another problem in that a large amount of energy is consumed to raise the temperature of the air, which is lower than the temperature of the combustion gas or the exhaust gas, to several hundred Celsius degrees.
Further methods for removing nitrogen oxide contained in air include an alkali absorption method using an alkali absorbing liquid (Patent Document 1), a reduction method using a reducer, and an absorption method using solid absorbents (Patent Document 2).
However, although the alkali absorption method is capable of absorbing nitrogen oxide of a low concentration, it also absorbs carbon dioxide contained together with the nitrogen oxide in air. Accordingly, there is a problem in that nitrogen oxide cannot be absorbed efficiently. As for the reduction method, since the reducer is oxidized by oxygen coexisting in the air, nitrogen oxide cannot be reduced efficiently. Further, as for the solid body absorption method, when nitrogen oxide is contained in air at a low concentration, the solid absorbents reach absorption breakthrough even though the solid absorbents have absorbed a few amount of nitrogen oxide. Accordingly, there is a problem in that the nitrogen oxide contained in the air cannot be absorbed sufficiently.
Furthermore, there is another method using a purification function of soil to remove nitrogen oxide contained in air at a low concentration. However, in case that the purification function of soil is used, there is a problem in that an extremely wide land is needed to secure a sufficient amount of soil to remove the nitrogen oxide.
A pressure required to pass gas through the above nitrogen oxide removal equipment is preferably low. Accordingly, the thickness of solid absorbents through which the gas passes is preferably as small as possible so far as the nitrogen oxide removal function thereof is not adversely affected.
However, to make the thickness of the solid absorbents through which gas passes small, the installation area for the solid absorbents needs to be increased along with increase in an amount of the gas to be treated in a unit time. Therefore, it is sometimes difficult to secure the installation space and this causes a problem. In particular, when the removal equipment needs to be constructed in a limited space although an amount of gas to be treated per unit time is large, there is a problem in that the removal equipment may not be installed because the installation space cannot be secured in the case where the removal equipment is installed specifically on a center divider, nearby a crossroads, under an elevated bridge, and in an air vent of a tunnel etc. in an urban area.
Furthermore, as the installation area for the solid absorbents increases, it is more difficult to pass gas through the solid absorbents at a constant flowing rate resulting in a problem of a phenomenon called “breakthrough” that the gas passes through the solid absorbents although the nitrogen oxide contained in the gas is not removed sufficiently by the solid absorbents, and this causes a problem of deterioration of the removal function.
In such nitrogen oxide removal equipment, the gas supplied to the nitrogen oxide absorption means is preferably humidified.
Further, a conventional nitrogen oxide removal equipment uses a humidifying means provided separately from the removal equipment to humidify the gas. Accordingly, the removal equipment and the humidifying means need to be interconnected with a piping duct for interconnection and this causes a problem in that installation spaces for the humidifying means and the piping duct for connection need to be secured before the installation of the removal equipment. There are also problems in that a pressure required to pass gas through the removal equipment is lost in the piping duct and the installation of the piping duct for interconnection is troublesome.    Patent Document 1: Japanese Laid-Open Patent Publication No. 211427/1998    Patent Document 2: Japanese Laid-Open Patent Publication No. 9957/1999    Patent Document 3: Japanese Laid-Open Patent Publication No. 259798/2001